Specify the Right Amount of Roof Insulation

ASHRAE recently updated its roofing insulation standards. New construction and major renovations should comply with these values.
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2) Select Appropriate Materials
“Insulation must work with the rest of the roof system. If you have the wrong kind, then you can’t properly adhere the membrane, and that’s bad for roof life,” Michelsen says. “Most roof systems don’t play well with others.”

Important characteristics of insulation include high resistance, low conductivity, ruggedness, water and solvent resistance, and high R-value per inch, explains Michelsen.

“There is not a perfect product out there, and cost is the driving factor. Roofs tend to be very large, and no one can afford hundreds of dollars per square foot of roof insulation,” he adds. “You must balance performance with cost competitiveness.”

The most common and current choices are polyisocyanurate board (roughly R-6 per inch), which is compatible with all membranes, and polystyrene foam (approximate R-values range from 3.8-5), which is compatible with single-ply membranes, says Michelsen.

“When reroofing comes around, it’s a concern that an owner might just reroof with what he had before,” Michelsen adds. “What was fine 10 or 20 years ago is probably no longer code compliant.”

Because code likely requires another layer or two of insulation, it’s important to know what your selection offers.

“Very often the roofing contractor selects the system, provides the owner with a bid, and pulls the building permit,” Michelsen says. “Know what’s in that specification and how it stacks up.”

3) Enhance Energy Performance
Seizing energy benefits is tricky, but being proactive will get you ahead of the curve.

“One of the dirty little secrets about roof insulation is that as we increase R-values and the amount of insulation, the amount of heat flow still increases but at a decreasing rate. Going from R-1 to R-2 reduces heat flow by 50%, but from R-19 to R-20 the reduction is maybe only 5%,” explains Michelsen.

“Insulation has a practical limit that you want to get to. After a certain value – which typically the codes dictate – you’re not getting the benefits you think you should by continuing to increase quantities,” he adds.

Reroofing is such a significant concern in terms of budget and capital that you want to make sure you’re doing the project right the first time, says Michelsen.

Whether that entails meeting the latest international and ASHRAE guidelines – or simply keeping local officials at bay by adopting their potentially outdated specifications – is up to you.

“Certainly it’s more beneficial in terms of energy conservation to add insulation up to the point of the most current guidelines, but that’s not always possible due to budgets or structural issues,” Smith says. “When I do a reroof project, I recommend updating to ASHRAE standards, because if the local code hasn’t made the shift yet, it soon will be. Then you’re safeguarding yourself for when it does and capitalizing on the energy efficiency properties in the meantime.”


Chris Curtland christopher.curtland@buildings.com is assistant editor of BUILDINGS.


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